More precisely, the continuous and automatic measurement of the concentration of such an electrochemical substance is embodied in a known way with the aid of a galvanic cell-type electrochemical sensor, which output delivers an electric intensity proportional to the concentration of the chemical substance dissolved in the liquid with which the sensor is in contact, through a semipermeable membrane, for example.
Unfortunately, and this is a phenomenon well-recognised by experts, such concentration sensors present two major drawbacks which impair their reliability:
The first drawback derives from the instability of the meter constant K of such a device between the electric intensity i it delivers and the concentration of the substance being observed. This constant effectively varies according to the time and temperature, which requires a continuous recalibration of the device so as to have perfectly reliable measurements.
Moreover, another drawback of this type of device results from the fact that, by virtue of its actual principle, it undergoes a certain amount of wear of its electrodes which leads it to a final state in which its indication is continuously nil, regardless of the concentration of the chemical substance monitored. This property constitutes a major drawback, as a nil indication of the sensor clearly constitutes an uncertainty due to the fact that it may be caused by both the absence of the substance sought-for and the definitive wear of the sensor.
In order to overcome this kind of difficulty, it is possible to periodically recalculate the instantaneous value of the meter constant K of the sensor by using the dosed additions method. This consists of injecting upstream in the pipe passed through by the liquid to be monitored known quantities of the chemical substance to be monitored and of deducing from this the instantaneous value of the meter constant K by studying the behavior of the response obtained at the level of the electrochemical sensor. However, such a method requires the use of a flowmeter, as the injector used, generally an electrolytic Faraday cell, making it possible to know the quantity of the substance injected but not its concentration. In order to know its concentration, it is necessary to use a flowmeter, which is a less reliable device having, in particular, heating systems or mobile elements and bearings, such as turbine bearings; such a device is also expensive, it alone representing more than 10% of the total cost of an automatic line concentration measuring installation.
Because of the above-mentioned reasons, there currently exists no reliable method for line measuring the concentration of a dissolved chemical substance which allows for the automatic calibration of the electrochemical sensor at periodic intervals.